1. Field of the Invention
The present invention relates generally to telecommunications systems and methods for avoiding channel interference between two cellular systems, and specifically to providing adaptive channel allocation for an indoor base station system to avoid channel interference with outside base station systems.
2. Background and Objects of the Present Invention
Cellular telecommunications is one of the fastest growing and most demanding telecommunications applications ever. Today it represents a large and continuously increasing percentage of all new telephone subscriptions around the world. Cellular networks have evolved into two different networks. The European cellular network uses the Global System for Mobile Communication (GSM) digital mobile cellular radio system. In the United States, cellular networks have traditionally been primarily analog, but recent advances have been incorporating digital systems within the analog networks. One such North American cellular network is the D-AMPS network, which is described hereinbelow.
With reference now to FIG. 1 of the drawings, there is illustrated a D-AMPS Public Land Mobile Network (PLMN), such as cellular network 10, which in turn is composed of a plurality of areas 12, each with a Mobile Services Center (MSC) 14 and an integrated Visitor Location Register (VLR) 16 therein. The MSC/VLR areas 12, in turn, include a plurality of Location Areas (LA) 18, which are defined as that part of a given MSC/VLR area 12 in which a mobile station (MS) 20 may move freely without having to send update location information to the MSC/VLR area 12 that controls the LA 18.
Each Location Area 12 is divided into a number of cells 22. Mobile Station (MS) 20 is the physical equipment, e.g., a car phone or other portable phone, used by mobile subscribers to communicate with the cellular network 10, each other, and users outside the subscribed network, both wireline and wireless. The MSC 14 is in communication with a Base Station (BS) 24. The BS is the physical equipment, illustrated for simplicity as a radio tower, that provides radio coverage to the geographical part of the cell 22 for which it is responsible.
With further reference to FIG. 1, the PLMN Service Area or cellular network 10 includes a Home Location Register (HLR) 26, which is a database maintaining all subscriber information, e.g., user profiles, current location information, International Mobile Subscriber Identity (IMSI) numbers, and other administrative information. The HLR 26 may be co-located with a given MSC 14, integrated with the MSC 14, or alternatively can service multiple MSCs 14, the latter of which is illustrated in FIG. 1.
The VLR 16 is a database containing information about all of the Mobile Stations 20 currently located within the MSC/VLR area 12. If a MS 20 roams into a new MSC/VLR area 12, the VLR 16 connected to that MSC 14 will request data about that Mobile Station 20 from the HLR database 26 (simultaneously informing the HLR 26 about the current location of the MS 20). Accordingly, if the user of the MS 20 then wants to make a call, the local VLR 16 will have the requisite identification information without having to reinterrogate the HLR 26. In the aforedescribed manner, the VLR and HLR databases 16 and 26, respectively, contain various subscriber information associated with a given MS 20.
The cell 22 can cover a wide geographical area, or alternatively, can consist of only a single building in which the Base Station System 24 is located within the building. For example, in high rise buildings, where such an indoor system 24 is likely to be implemented, the external channel interference to and from other outdoor Base Station Systems (not shown) can be high because the effective antenna heights are high. This is due to the fact that there is typically a direct line of sight between the indoor BS 24 and the outdoor BS (not shown), e.g., there are no buildings or hills obstructing the signals from each of the BSs. Therefore, in order to minimize channel interference, the indoor BS 24 must choose the best frequency for each traffic channel in the indoor system.
Currently, for an indoor system 24, the channels used for traffic channels are selected based on a scanning receiver that examines all of the up-link frequencies, e.g., the frequencies used by the MSs 20 to transmit voice and data to the BS 24, and chooses the frequencies with least interfering power. The up-link frequencies are continuously scanned and sorted according to least channel interference by the scanning receiver. Many MSs 20 can also scan the down-link frequencies, e.g., frequencies used by the BS 24 to transmit voice and data to the MSs 20, but the scanning is much slower and therefore, not as accurate.
For example, if the indoor cellular system 24 has 12.5 MHZ (416 channels) with twenty-one of those channels being control channels, e.g., channels used only for signaling, and the indoor system 24 needs twenty-five channels, a scan receiver (not shown) will scan 395 frequencies (416-21 control channels) on the up-link, choose the twenty-five frequencies with the minimum interference level and allocate those frequencies for the indoor traffic channels. Using both the scanning receiver for the up-link and the MS 20 for the down-link, the indoor base station system 24 sorts the twenty-five channels and allocates each call to the least noisy available channel.
However, this solution does not accurately account for the down-link interference, which typically is greater than the up-link interference, because on the down-link frequency, all three time slots (for D-AMPS systems) are transmitted even if only one conversation is occurring, e.g., only one MS 20 is connected on that frequency. This can result in up to three times the possibility for interference on the down-link frequency than on the up-link frequency, e.g., because the MS 20 will occupy only one time slot. In addition, the power (strength) of the down-link frequency is currently not able to be controlled, while many MSs 20 have, for example, 24 dB of power control for the up-link frequency. This results in the average interference level on the down-link being higher than the average interference level on the up-link. Furthermore, in high rise buildings, cellular phone users tend to go near a window in order to improve voice quality. However, in some cases, this may only increase the interference because they may be in line of sight with an outdoor interfering base station, which, of course, results in higher interference on the down-link.
It is therefore an object of the invention to provide adaptive channel allocation for indoor cellular systems based on both the down-link and the up-link channel interference.